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Credit facilities (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2013 | Dec. 31, 2012
Line of credit facility and other debt [Abstract] |  |  |  |
Letters of credit at end of period | $ 36.0 |  |  |
Long-term debt [Member] | Commercial paper revolving credit agreement [Member] | MDU Resources Group, Inc [Member] |  |  |  |
Line of credit facility and other debt [Abstract] |  |  |  |
Facility limit | 125.0 |  |  |
Amount outstanding, end of period | 78.9 | [1] | 76.0 | [1]
Expiration date | Oct. 04, 2017 |  |  |
Long-term debt [Member] | Commercial paper revolving credit agreement [Member] | Centennial Energy Holdings, Inc [Member] |  |  |  |
Line of credit facility and other debt [Abstract] |  |  |  |
Facility limit | 500.0 |  |  |
Amount outstanding, end of period | 75.0 | [1] | 217.0 | [1]
Expiration date | Jun. 08, 2017 |  |  |
Long-term debt [Member] | Revolving credit agreement [Member] | MDU Resources Group, Inc [Member] |  |  |  |
Line of credit facility and other debt [Abstract] |  |  |  |
Letters of credit at end of period | 0 |  |  |
Option to increase borrowings, maximum amount | 150 |  |  |
Long-term debt [Member] | Revolving credit agreement [Member] | Intermountain Gas Company [Member] |  |  |  |
Line of credit facility and other debt [Abstract] |  |  |  |
Facility limit | 65.0 | [2] |  |
Amount outstanding, end of period | 3.0 |  | 26.2 |
Letters of credit at end of period | 0 |  |  |
Expiration date | Jul. 13, 2018 |  |  |
Option to increase borrowings, maximum amount | 90 |  |  |
Long-term debt [Member] | Revolving credit agreement [Member] | Centennial Energy Holdings, Inc [Member] |  |  |  |
Line of credit facility and other debt [Abstract] |  |  |  |
Letters of credit at end of period | 0 |  |  |
Option to increase borrowings, maximum amount | 650 |  |  |
Short-term debt [Member] | Revolving credit agreement [Member] | Cascade Natural Gas Corporation [Member] |  |  |  |
Line of credit facility and other debt [Abstract] |  |  |  |
Facility limit | 50.0 | [3] |  |
Amount outstanding, end of period | 11.5 |  | 2.0 |
Letters of credit at end of period | 2.2 | [4] |  |
Expiration date | Jul. 09, 2018 |  |  |
Option to increase borrowings, maximum amount | $ 75 |  |  |
[1] | Amount outstanding under commercial paper program
[2] | Certain provisions allow for increased borrowings, up to a maximum of $90 million.
[3] | Certain provisions allow for increased borrowings, up to a maximum of $75 million.
[4] | The outstanding letter of credit, as discussed in Note 19, reduces the amount available under the credit agreement.

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- Definition Letters of credit. + References No definition available. + Details
Name: | mdu_LettersOfCredit
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The option to increase borrowing capacity to the stated maximum amount, at the option of the company, on stated conditions. + References No definition available. + Details
Name: | mdu_OptionToIncreaseBorrowingsMaximumAmount
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_LineOfCreditFacilityAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount borrowed under the credit facility as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
Name: | us-gaap_LineOfCreditFacilityAmountOutstanding
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Date the credit facility terminates, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
Name: | us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant